[icecast] vorbis bitrates - offtopic

Jack Moffitt jack at xiph.org
Sat Oct 20 15:32:45 UTC 2001

> vorbis_encode_init( &vorbisInfo, 2, 44100, 96, 96, 96);
> which by all reasons should generate a 96 kb/s stream, as all
> max_bitrate, nominal_bitrate and min_bitrate are set to 96. Strangely
> enough, the generated stream's bitrate (according to XMMS and WinAmp)
> varies between 49 and 59.

Currently min and max are ignored by the library, since bounded bitrates
aren't going in until rc3.

Also, if you pick 96kbps, that picks the 96kbps mode (if there is a
96kbps mode.  It might be picking the 80kbps mode.  You'll have to look
at the vorbisml to figure out which mode that's actually using).  In any
case, unlike mp3, which must be at a fixed bitrate, Vorbis won't use
more bits than it needs.  So often you'll get streams that are less or
more than what you picked.  That's the nature of VBR.  It tries to keep
the average at 96.  What was the average of the entire encoded piece?

> If I try to set the same bitrate to a low 24, I never get a stream below
> about 44 kb/s.

In rc2 there are no modes below 64kbps.  So this sounds correct.

> Could anyone enlighten me on these strange behaviours? It would be quite
> important to be able to produce a stream that still fits 33.6 kb/s modem
> users...

If you downsample to 22.05kHz, that mode will produce much smaller
files.  It will also get way smaller if you use mono input rather than
stereo.  Vorbis currently does not do this processing for you.  I
believe the lowest mode that 1.0 will support at 44.1kHz is 48kbps.

In any case, you really should re-ask this question on the Vorbis list,
where many more people can offer you input.

Once more tunings are in, and the tools do things like downsampling and
downmixing for you, you'll probably not notice a big difference from
what you are used to, except now and then if a song is easy to encode,
it'll use less bits than you asked.  For streaming applications you'll
also be able to specify how much the bitrate can vary, how low and high
it can go, etc.  That will be quite useful.  

And finally, you want VBR because it sounds better.  People just aren't
used to VBR yet.


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